Southwest Michigan grape scouting report for June 13, 2017

Juice grape bloom is open while vinifera are in the pre-bloom period; brief review of available spray materials; instructions on grape berry moth model.

Webbing and damaged flower parts in blooming grape. Photo by Keith Mason, MSU.

Webbing and damaged flower parts in blooming grape. Photo by Keith Mason, MSU.

Growing degree-day (GDD) accumulations

Station

GDD50, June 12

Date of wild grape bloom

GDD47 since wild grape bloom

Berrien Springs

749

May 26

359

Benton Harbor

807

May 27

334

Lawton

733

May 27

349

Freeze damage

Recovery from the May 8 freeze continues to be as variable as damage was the day of. Some damaged hybrid blocks have made a full recovery and other blocks will bear little to no crop. In many juice grape vineyards with significant primary shoot loss and strong recovery of secondary shoots, there will be two bloom periods, and two flushes of crop, one ripening ahead of the other. This complicates crop management and disease management, extending the period of vulnerability to fruit infections by several weeks.

Where a majority of the primaries have been lost, and where secondary or tertiary shoots do not appear to have fruiting potential, growers should consider a reduced fungicide schedule to cover only leaf and shoot infections. Consult your crop insurance provider before making this decision.

Development

In Concord and Niagara, bloom is under way. Some early vineyards have already finished bloom and are setting fruit now. Depending on the variety and location, wine grapes are anywhere between open bloom and the pre-bloom period. Shoot growth continues to be strong, and recovering secondary shoots after the frost are catching up with primaries.

Disease

Disease pressure appears to be light so far, due to relatively dry weather and good pre-bloom disease management - our scouting sites are looking pretty clean. The one exception is phomopsis on shoot bases, due to the moisture during early shoot growth. There are also a few areas where we’re seeing downy mildew and black rot on leaves.

Black rot infections beginning to show on Concord leaves.  Photo by Brad Baughman

Black rot infections beginning to show on Concord leaves. Photo by Brad Baughman, MSU.

From now until two to four weeks after bloom, the fruit and rachises need to remain covered for all of the major fungal diseases - black rot, phomopsis, downy mildew and powdery mildew and, where relevant, botrytis or anthracnose. Tank mixes and pre-mixes will be needed for effective disease control during this period. This is the time of year to use the full rate of most materials, especially the pre-mixes. There are a lot of choices. 

Some of the possibilities are:

  1. SI material with EBDC or Ziram
  2. Quadris Top or Topguard EQ with EBDC
  3. EBDC with Sulfur
  4. Pristine or Merivon alone, but never repeat in sequential sprays
  5. Luna Experience, which covers everything but Downy
  6. Revus Top, which is strong on Downy mildew, and requires the full rate to cover for Powdery mildew
  7. Vivando or Quintec with EBDC or Ziram

Of these, only 4 and 5 offer strong control of Botrytis. In susceptible varieties, work in Vangard, Inspire Super, Switch, Scala, Elevate or Rovral during bloom or shortly afterwards to protect against Botrytis latent infections. Botrytis risk is reduced if weather continues to be warm and dry during bloom. See Michigan State University Extension bulletin E0154 “Michigan Fruit Management Guide” for additional options.

Insects

Rose chafers are showing up in vineyards. Scout for these beetles that feed on leaves, especially after the next significant rain. Use contact insecticides after bloom if control becomes necessary. Meanwhile, both species of leafhoppers (grape leafhopper and potato leafhopper) are being seen, still in pretty low numbers. Don’t neglect scouting for these pests. Check the underside of leaves in the shaded part of the canopy. If you start to see serious damage, treat with a neonicotinoid after bloom.

Grape berry moth presence in clusters is still very low. The flight is still strong, depending on the site. Trap catches varied between one and 331 moths per trap this week. As usual, we are catching the most grape berry moth in Van Buren County vineyards.

Grape berry moth model

You probably won’t need to spray for 2nd-generation grape berry moth until after our next scouting report. But if the weather gets hot enough, it might be time to spray for grape berry moth eggs and larvae just before.

Therefore, instructions on determining spray timing follow:

Your goal should always be to have poisons on the leaves and fruit when grape berry moth are eggs and larvae since that is when they are most vulnerable. We have broadly two groups of insecticides for grape berry moth for which the timing varies slightly.

One group is the ingestion poisons - excellent materials Altacor, Belt or Intrepid, and good materials BT, Entrust or Delegate. If you’re going to use these materials, you need to get very good coverage (no skipping rows, Concord growers!) and apply as close to 810 GDD47 after wild grape bloom as possible.

The other group is the contact poisons - excellent materials Imidan, Sevin, Gladiator, Danitol and Hero, or the rest of the Pyrethriods which are also good. Really good coverage is less important with these materials, and you’re shooting for 910 GDD47 after wild grape bloom. Wild grape bloom averages were around May 26-27 this year, which could be a little bit off from your farm depending on where you’re at.

So when do you spray? In the previous two years, 810 GDD47 was right around July 4, and 910 GDD47 was just a few days later. To figure out when it’s happening this year, you can check the grape berry moth model on MSU Enviroweather. If you get the timing right, you’ll be applying just as the majority of the females are laying eggs on the clusters.

If you have a vineyard with a history of very light berry moth pressure, a low trap catch and with little to no larvae in the blossoms and clusters so far, you can do just a border spray for this upcoming spray window.

As always, thanks for reading! Don’t be afraid to contact either of us for questions, comments or suggestions.

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